There are a variety of methods known for generally determining the health of gas turbine engines, particularly aircraft engines. General engine condition, as well as an indication of engine life expectancy and need for overhaul, is provided by trending systems which utilize engine parameters such as various temperatures, pressures and control parameters associated therewith to determine current engine operating condition and impute engine health. However, such systems do not recognize isolated events within the engine which can be indicative of severe engine distress or impending engine component failure from a particular causal event.
Certain engine conditions can be determined visually, such as through borescopes, without tearing down the engine. As examples, severe blade erosion (high temperature corrosion), loss of abradable seal segments, or excessive rubbing can frequently be detected by borescope inspection methods. Additonally, periodic teardown of an engine allows inspection of far more components, much more reliably. Because engine teardown is such a complex and expensive proposition, various schemes are employed to determine when engine teardown should be performed. Tearing engines down too frequently is, of course, an extreme waste of time and money. Failure to tear down an engine when it may have problems could result in engines malfunctioning while in use. Because of the complexity and expense involved, any improvement in diagnostic methodology is of great value.
In the past decade, monitoring of the electrical characteristics of gas flowing through a jet engine has been studied as a possible indication of engine deterioration. Apparatus disclosed in U.S. Pat. No. 3,775,763 utilizes an electrostatic probe positioned in the exhaust of the jet engine, such as through the tail pipe wall. Abnormal conditions were thought to be coupled with small particles striking the probe and causing spikes of ion current of a relatively large magnitude. Subsequently, as reported by Couch, R. P.: "Detecting Abnormal Turbine Engine Deterioration Using Electrostatic Methods", Journal of Aircraft, Vol. 15, Oct. 1978, pp 692-695, it was theorized that the signals did not result from individual particles of metal hitting the probe, but rather that the signals were indicative of Trichel pulses (a form of repetitive corona discharge) created by high potential pockets of excess charge. A probe set including circular insulated segments within the gas turbine engine tail pipe and a triangle of wire extending through the tail pipe exhaust gas path were developed. An oscillogram of a charge pocket signal caused by a rub, with the Trichel pulses filtered out, sensed by the ring and grid probes are shown in the article. With these probes, a normalized count of large signals (probe current, or voltage developed across an impedance, in excess of a threshold magnitude) over a period of time definitely correlated with impending engine component malfunctions or severe deterioration. As reported in the aforementioned article, however, the use of normalized counts of large magnitude signals from the ring and grid probe was thought to provide reliable prediction of only two out of three gas-path failures, at best, and distinction between possible causes thereof was highly experimental, as described below.
In the article, it is postulated that signals indicative, separately, of the plus pulse count and minus pulse count above a preset threshold from the ring probe and from the grid probe, as well as signals indicative of the area above a preset threshold in both the plus and minus directions (eight different signals in total), will provide signatures unique as to engine section, such as compressor, combustor, and turbine. The article reports that five failures had been observed to date with a unique distribution of counts. The attempt to develop unique engine section signatures from the count and size of plus and minus signals from the two different probes at the same station was abandoned.
In pursuit of a more satisfactory manner of acquiring and utilizing information related to electrostatic activity in the gas path of gas turbines, consideration was given to waveshapes of signals developed from electrostatic probes disposed in the gas stream of an engine. This effort evolved into techniques for acquiring waveshapes of electrostatic activity and analyzing them for correlation with causal engine events, as disclosed in a commonly owned, copending U.S. patent application entitled "Waveform Discriminated Electrostatic Engine Diagnostics", Ser. No. 454,124 filed contemporaneously herewith by the inventors hereof. As disclosed therein, it was learned that the diagnostic equipment could itself provide, in response to faults in the equipment, signal waveshapes which are quite similar to waveshapes derived from electrostatic activity in the gas path. Naturally, the classification of a signal by its waveshape as being within a category correlated with an engine event related to abnormal engine component wear (such as erosion and rubbing), requires discrimination of each signal category from other categories, including those produced by the diagnostic equipment itself. Naturally, the value of such a classification system is diminished if unable to discriminate waveshapes related to correlated causes from other, similar waveshapes.